1. Field of the Invention
Appliances for tensioning ligaments, especially articular ligaments, of a human or animal body are used for tensioning soft tissue structures in controlled manner in the case of an operation for the replacement of degenerated joints by artificial joints. Known examples thereof are the replacement of hip joints and knee joints and also the replacement of vertebrae by artificial joints.
2. Description of the Related Art
For the success of an operation for the replacement of a damaged joint by an artificial joint, it is important that the passive structures stabilising the artificial joint, that is to say the ligaments and the capsule which surrounds the joint, are correctly adjusted.
The relatively recent literature on the subject of joint replacement describes the importance, for operative success, of quantitative assessment of the capsule/ligament structures (cf., for example, “An In-Vivo Biomechanical Analysis of the Soft-Tissue Envelope of Osteoarthritic Knees” in “The Journal of Arthroplasty”, Vol. 19, 2004). The findings collated in that document result in an operative technique which in general terms starts from the basis of the ligaments' being of constant stiffness.
However, in the investigations carried out therein, considerable variations of up to 30% were recorded in the stiffness of the capsule/ligament apparatus. The mechanical properties of the capsule/ligament apparatus accordingly vary greatly between individuals. In addition, these properties are dependent on the state of disease of the joint and on the end state of the capsule/ligament apparatus, especially—in the case of the knee—on the state of the posterior cruciate ligament. Accordingly, only approximate information can be provided for the distraction forces that are to be used. Differences arise as a result of, inter alia, the different leg weight to be moved, the age of the patient and the nature of the disease. Especially strong structures are to be expected in relatively young patients with post-traumatic arthrosis. In the case of senile arthroses and rheumatic diseases, the quality of the structures can be expected to drop.
If stability of the joint in flexion is not achieved during or after the operation, the result is that the kinematics of the artificial joint are not optimal. A known example thereof is in the case of replacement of the knee joint by an artificial joint, where an upper leg component slides forward on a lower leg component as the knee is increasingly flexed, which is exactly the opposite of the natural sequence of motion of the knee joint.
Optimum joint kinematics are to be expected in the case of ligament structures which are almost isometrically stressed. In terms of operative technique, this is achieved by establishing joint stability in two positions. In the case of replacement of a knee joint, the assessment is made in 90° flexion and in extension. Optimum stability of the joint can be found from a force-displacement characteristic curve in simple manner. At said “stability point”, all the structures are only just lightly tensioned but not yet elastically extended. As already mentioned hereinbefore, that stability point is patient-specific. This consequently leads to its being necessary to record a force-displacement characteristic curve for the joint being operated on, so that the necessary information can be obtained for determination of the stability point. Because of the demands made of an operative instrument, it has not hitherto been possible to make a recording of such a kind.
According to the prior art, relatively simply constructed appliances are used for tensioning ligaments, some of which are also provided with a mechanical force indicator, for indicating the force with which a joint is distracted, and with an indicator of the distraction displacement, that is to say the distance through which two joint compartments are distracted. An appliance of such a kind for tensioning ligaments is shown, for example, in WO 00/78225 A1.
A further appliance for the distraction of ligaments stems from the Applicant. Said appliance consists of two distraction elements actuatable by means of two mutually independent handles by way of a helical compression spring. Excursion of the distraction elements is accomplished by means of a force produced by compression of the helical compression spring using the handle in question. In addition to the force exerted on the particular distraction element, the displacement that the element in question undergoes can also be read off on the appliance so that a corresponding force-displacement relationship can be established. Both the force and also the displacement that the distraction element undergoes are indicated on a scale mounted on the appliance and read by the operating surgeon.
Using the afore-mentioned appliances for tensioning ligaments, especially articular ligaments, it is not possible to record a precise force-displacement characteristic curve having a high point density. In addition to inaccuracies in reading, caused by the measurement values' being read off by the operating surgeon or assisting staff, it is not possible to produce the point density necessary for an exact characteristic curve because the duration of the operation would be disproportionately prolonged as a result.